In recent years, the electrophoretic display method, which employs the electrophoresis of charged particles (electret particles), has been attracting attention as the most promising technology for a next-generation display apparatus. However, this technology still has many problems including the shape of charged particles, small and unstable charge potential (ζ potential), secondary aggregation or sedimentation of electrophoretic particles, inadequate deletion of previously displayed images, unsatisfactory response speed, and the like.
Patent Literatures 1 and 2 disclose electret powders used for the above purpose.
Patent Literature 1 discloses negatively-charged fine particles, which are formed by adding a resin serving as an electron trap to each core resin of spherical ultrafine particles of 1 to 10 μm in diameter produced by polymerizing a polymeric fine particle material, and irradiating the spherical ultrafine particles with a 10-300 kGy electron beam to obtain electret particles, wherein the core resins are colored with an arbitrary color (claim 1).
Patent Literature 2 discloses use of negatively-charged color particles, which are formed by adding pigment, a material serving as an electron trap, etc., to a polymeric fine particle monomer material to form spherical ultrafine particles of 5 to 10 μm in diameter by way of suspension polymerization, emulsification polymerization, dispersion polymerization or the like, and irradiating the ultrafine particles with a 10-50 kGy electron beam before heating the particles at 90° C. to 110° C. for ten and several minutes or irradiating the ultrafine particles with a 10-50 kGy electron beam at 90° C. to 100° C., thereby obtaining negatively-charged electret fine particles, wherein the particles have −50 to −100 mV ζ potential, and are colored with an arbitrary color (claim 10).
However, when these electret particles prepared by conventional polymerization methods are used as the electrophoretic particles for a large-screen display, their particle size is too small for the large display and can be a disadvantage. For example, when such small particles are used to carry out image display, the image is displayed in the form of dots, and there is a possibility that the particles have gaps therebetween on the screen. For this reason, coarse powder is more suitable than fine particles as the electrophoretic particles for use in a large display. However, coarse powder that ensures a desirable response performance has not been developed yet.
Therefore, there has been a demand for the development of a method for efficiently producing electret coarse powder useful as the electrophoretic particles for a large display.